In modern integrated circuits, semiconductor devices are formed on semiconductor substrates, and are connected through metallization layers. The metallization layers are connected to the semiconductor devices through contacts, also referred to as contact plugs. Also, external pads are connected to the semiconductor devices through the contacts.
FIG. 1 illustrates conventional contacts for connecting the semiconductor devices to the metallization layers. Metal-oxide-semiconductor (MOS) device 4 is formed at the surface of semiconductor substrate 2. Inter-layer dielectric (ILD) 10 is formed over MOS device 4. Contacts 6 are formed in ILD 10 to connect source and drain regions 14 and gate 16 of MOS device 4 to metal lines 7, which are in metallization layer 8. Typically, the formation of contacts 6 includes forming openings in ILD 10, and then filling the openings with tungsten plugs. A single damascene process is then performed to form metallization layer 8.
Due to tungsten's relatively low conductivity, copper is increasingly used to replace tungsten for forming contacts. Copper has a significantly higher conductivity (about 0.596 106/cm·Ω) than tungsten (about 0.189 106/cm·Ω). Therefore, the resulting resistances of copper contacts are significantly lower than that of tungsten contacts. However, copper has greater diffusion distance. Under elevated temperatures, copper tends to diffuse into the underlying source/drain regions 14 and gate electrode 16, forming copper silicide therein. This adversely increases the leakage currents, and may even cause device failure. To solve this problem, diffusion barriers 12 were formed on the bottoms and sidewalls of the contact openings before filling copper. The bottom portions of diffusion barriers 12 prevent copper from diffusing into the underlying source/drain regions 14 and gate electrode 16.
With the increasing down-scaling of integrated circuits, the above-discussed processes experience shortcomings. While the horizontal dimensions, such as the width W of contacts 6, are continuously shrinking, the thickness of ILD 10 is not reduced accordingly to the same scale as width W of contact plugs 6. Accordingly, the aspect ratio of contacts 6 continuously increases. The high aspect ratio results in difficulties in the formation of contact barriers 12, which in turn causes bottom portions of the contact barriers to be discontinuous. Copper thus adversely diffuses into the underlying source/drain regions and gate electrode.
Accordingly, what is needed in the art is a new contact barrier structure and formation methods for solving the above-discussed problems.